This invention relates to stable, pumpable, solvent-free polyampholyte latices comprising water and an amphoteric copolymer colloidally dispersed therein. It also relates to a process for preparing these latices. It further relates to their use as paper-making aids such as retention and drainage aids.
Interest in the preparation and use of water-soluble copolymers has increased due to government regulations concerning pollution abatement. These copolymers may be non-ionic, cationic, anionic, or amphoteric, depending upon the nature of the water-soluble or water-dispersible monomers used in their preparation. They are useful in the paper, petroleum, water treatment, and mineral processing industries. However, in order to obtain performance characteristics comparable with those of water-insoluble copolymers, it is necessary to use high molecular weight copolymers, which has presented handling problems. These high molecular weight, water-soluble copolymers are currently available in four forms (i.e. solids, aqueous solutions, aqueous emulsions, and aqueous dispersions), all of which have disadvantages in the desired application areas.
Use of the solid copolymers directly is impractical and would require the use of expensive equipment for dissolution and constant proportioning. It is more efficient and practical to use aqueous solutions, which for economic reasons should have high solids contents. The preparation of such solutions from the solid copolymer is difficult, requiring excessive dissolution times since the copolymers, when added to water, form gels (i.e. water-swollen particles whose exterior portions are covered with a gelatinous water-polymer mixture which retards movement of water into the particle and reduces the particle's effectiveness in an application area). Further these high solids solutions are impractical due to their very high viscosity and the difficulty of handling (i.e. pumping) them.
Dilute aqueous solutions of high molecular weight copolymers can be prepared directly by known solution polymerization techniques, but these are also impractical to use due to their low solids content (e.g. about 5% copolymer by weight). In addition, even at these low solids contents, the solutions have a high viscosity making them also difficult to handle.
To overcome the above difficulties and to still maintain the convenience of a liquid product, the trend has been to the use of aqueous emulsions resulting from water-in-oil (W/O) heterogeneous polymerization techniques. These techniques are well-known and result in high solids, low viscosity emulsions containing the preferred high molecular weight copolymers. However, these emulsions, although pumpable, require special handling. They tend to separate into a polymer-poor oil phase and polymer-rich phase, and they are flammable due to the presence of organic solvents (e.g. toluene or paraffinic hydrocarbons) which are used to form the continuous water-insoluble (oil) phase. They have several other serious drawbacks. First, unlike the emulsions resulting from conventional emulsion polymerizations using oil-in-water emulsifiers, they contain large amounts of water-in-oil emulsifiers, typically amounting to about 10-20% by weight of the continuous phase. Second, they result in increased water pollution when used directly in application areas such as paper-making since neither the emulsifier nor solvent are retained to any significant degree. Finally, most of the emulsions require an "inversion step" after dilution.
To overcome the handling difficulties involved in the direct use of W/O aqueous emulsions, it has been suggested that aqueous dispersions containing copolymer, a low HLB surfactant, and a water-soluble organic solvent, used as a carrier, be prepared (see U.S. Pat. No. 4,176,107). These dispersions can then be rapidly dissolved in water. However, the preparation of these dispersions from the aqueous W/O emulsions involves the additional step of removing the oil phase either prior to or after addition of surfactant and carrier (typically a polyalkylene glycol). Even though this eliminates the handling difficulty due to the flammable solvent, it still suffers from serious drawbacks in that surfactant and organic solvent are present in the final solution.
All of the above difficulties and drawbacks would be obviated if stable, pumpable, high solids aqueous latices containing amphoteric copolymers could be prepared, without the use of emulsifiers or solvents, which had the performance characteristics of the above high molecular weight solution copolymers and which could be used directly without further processing.
It is an object of this invention to provide such latices, specifically polyampholyte latices useful in paper-making applications.
We have discovered that the performance characteristics of the solid high molecular weight copolymers previously discussed can be achieved with low viscosity, easily pumpable latices, which do not contain surfactants, emulsifiers or solvents, and, as such, overcome the drawbacks and disadvantages of the prior art W/O emulsion and solution copolymers. These latices, which are true colloids even in dilute solution, demonstrate unexpectedly good performance characteristics in view of the traditional thinking that colloidal copolymers do not perform as well as their solution counterparts in wet end applications. By proper design of the latex copolymer with respect to monomer composition, as well as charge density and charge ratio, however, high molecular weight, low viscosity products with excellent performance characteristics can be obtained.